The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
It is often desirable to extend the release time of an injected drug to increase its duration of action, or to reduce its toxic effects. Formulations that are readily soluble in the body are usually absorbed rapidly and provide a sudden burst of available drug as opposed to a more desirable and gradual release of the pharmacologically active product. A variety of attempts have been made to provide controlled and extended release pharmaceutical compounds, but have not succeeded in overcoming all of the problems associated with the technology, such as achieving an extended release time, maximum stability and efficacy, reduced toxicity, maximum reproducibility in preparation, and the elimination of unwanted physical, biochemical, or toxicological effects introduced by undesirable matrix materials.
Oxytetracycline is a widely used and useful antibiotic for treating various infections in mammals. In particular it is used for treating and preventing respiratory infections in domestic animals. There are significant costs associated with repeated administrations through conventional means.
Tilmicosin is a macrolide antibiotic with two tertiary amines. It has a long tissue half-life and is effective against a broad range of bacteria and is used to treat respiratory diseases in cattle. At elevated levels tilmicosin is cardiotoxic and its use in sensitive species such as cats, goats, pigs and horses has been avoided almost entirely due to safety reasons. The commercial product, Micotil® (Eli Lilly & Co., Indianapolis, Ind.), is a solution of the di-phosphate salt and is described in U.S. Pat. No. 5,574,020. This formulation is effective in cattle, but the antibiotic is released rapidly and results in toxicity in many species, including dogs and cats.